Maleic Anhydride is an important raw material used in the manufacture of alkyd and unsaturated polyester resins and it is also a versatile intermediate for the chemical synthesis of butanediol and tetrahydrofuran. It is produced by partial oxidation of hydrocarbons, typically n-butane, over a vanadium-phosphorus-oxygen (VPO) catalyst contained in a tubular fixed bed or in a fluid bed reactor. In both types of reactor the substantial exothermic heat of reaction is removed with production of steam.
To reduce risks of explosivity, the effluent gas from the reaction typically contains only a small amount of maleic anhydride, i.e. from 0.6 to 1.2 percent by volume, together with other components, mainly nitrogen and residual oxygen, and with oxidation by-products, including carbon monoxide and dioxide, water, acetic acid, acrylic acid and other minor impurities.
In the conventional process, first used in the commercial applications, the maleic anhydride present with low concentration in the gaseous stream which leaves the oxidation reactor, after adequate cooling and in some cases partial condensation, is recovered by absorption in water, where the corresponding maleic acid is formed.
The conventional process has some major disadvantages, as for example, a low yields of recovery, owing to conversion of maleic acid to fumaric acid, an high energy consumption due to the hydration of all the maleic anhydride to maleic/fumaric acid and the subsequent dehydration to the anhydride form, a discontinuous steam demand, high manpower and maintenance requirements and the production of high amounts of waste water effluent.
An alternate process based on absorption of maleic anhydride with use of an organic solvent has been proposed.
U.S. Pat. No. 2,574,644 can be considered the first application mentioning the use of organic solvent, in particular dibutyl-phthalate (DBP), as a selective absorbent for maleic anhydride. The patent teaches that, thanks to the great solubility of maleic anhydride in, the solvent even at low temperature, the maleic anhydride can be recovered in a column using DBP as absorption medium even at moderate conditions of temperature and pressure. Subsequently the maleic anhydride is removed from the solvent by stripping under vacuum conditions and elevated temperature.
GB patent 727,828 and U.S. Pat. No. 2,942,005 refers to the use of dibutyl phthalate for simultaneous absorption, from a mixture gas containing maleic and phthalic anhydride, of both anhydrides which is followed by vacuum stripping and separation of the products by fractionation (GB 727,828) or by crystallization (U.S. Pat. No. 2,942,005).
GB patent 763,339 and GB patent 768,551 represent improvements of the organic solvent recovery system, where the rich solvent is subject to two subsequent stripping operations at controlled conditions of pressure and temperatures.
U.S. Pat. No. 3,818,680 discloses the use of an alkenyl or alkyl substituted succinic anhydride as absorption liquid in a column having a number of theoretical trays from 5 to 15 and operating at temperature between 65 to 125° C.
U.S. Pat. No. 3,891,680 describes the use of dialkyl phthalate esters, with lower vapor pressure than DBP, capable to prevent carryover loss of solvent out of the gas scrubbing column and out of the vacuum stripper. Preferred solvents are in the group of dihexyl ortophthalates.
U.S. Pat. No. 3,948,623 relates to a process for separating maleic anhydride from a gaseous mixture by absorption into an organic solvent fed to an absorption column, where the absorption heat is removed through the cooling of a recirculation solvent stream withdrawn from the lower part of the column and recycled in an intermediate section of the column.
U.S. Pat. No. 4,071,540 discloses the absorption of maleic anhydride by countercurrent contacting in a column, by using liquid solvent comprising polymethyl-benzophenones.
U.S. Pat. No. 4,118,403 represents a further application of phthalate esters, preferably dibutyl phthalate, as absorption liquid, in which the addition of some amount of phthalic anhydride permits a better control of the temperature at the bottom of the vacuum stripper, preventing an excessive decomposition of the dialkyl phthalate.
U.S. Pat. No. 4,314,946 refers to a process to recover maleic anhydride from reaction gases, by using a different group of organic solvents, specifically dialkyl esters of hexahydrophthalic acid, tetrahydrophthalic acid, methyl tetrahydrophthalic acid or methyl hexahydrophthalic acid.
EP patent 0459543 discloses an improved process to recover maleic anhydride with an organic solvent, specifically dibutyl phthalate, characterized by a reduced formation of maleic and fumaric acid due to a substantial removal of absorbed water from the enriched absorbent, by its stripping with a low humidity gas or by contacting it with a water absorbent of several types, including zeolites.
Chinese patent CN 1062344 refers to the use of different type of organic solvents, like phthalic dioctyl ester or dewaxed oil from refinery, to recover maleic anhydride from the catalytic oxidation of hydrocarbons.
Japanese patent JP5025154 moreover teaches the uses of another organic solvent consisting of tetrahydrothiophene 1,1-dioxide.
U.S. Pat. Nos. 5,631,387, 6,093,835 and 6,921,830 consider the removal of accumulated polymeric impurities and other contaminants from the recirculating solvent, the first by using water washing the second by using aqueous alkali solution and the third by distillation under sub-atmospheric pressure.
US patent application 2009/0143601 presents a process for preventing fumaric acid deposits in the preparation of maleic anhydride, in which the organic solvent, preferably a phthalate ester, is catalytically hydrogenated before being recycled to the absorption column.
Patent WO 2009/121735 also refers to a process for separating fumaric acid in the recirculating absorbent liquid, by means of a controlled precipitation as solid, by cooling or by evaporative concentration.
Most of the above mentioned patents applied in around 60 years do not concern the art of the absorption of the maleic anhydride into the organic solvent, which is supposed to be a very simple unit operation.
The few exceptions are as follows:
                U.S. Pat. No. 3,948,623 explains that, in order to maintain a proper temperature profile in the absorption column, the cooling of the absorbent liquid sent to the top of the column, as was done in the prior art processes, is not sufficient. An additional heat removal is necessary: preferably the heat shall be removed by a cooled recycle loop of absorbent, withdrawing the liquid stream from the lower part of the column, cooling the stream and then recycling the resulting cooling absorbent to the column.        U.S. Pat. No. 4,314,946 briefly mentions under examples 1 and 3 that, in order to cool the reaction gas inside the absorber down to 60-65° C., which represents the right temperature of the gas leaving the absorber to have an efficient recovery of maleic anhydride, the absorption column shall be divided in two columns in series with a gas cooler in between. Of course it is a complex and expensive solution.        EP Patent 0459543, on the other hand, explains that the enriched liquid absorbent, containing not negligible amount of water, absorbed from the oxidation reaction gas, before to be sent to the vacuum stripping column, where the maleic anhydride is recovered from the rich liquid absorbent, should be subject to a drying step.        
In spite of the lacking of interest of the patents literature in this subject, as a matter of fact the issues related to the absorption in an organic solvent, and more specifically in the dibutyl phthalate, which is the most important and common organic solvent in the industrial applications, are somewhat complex and some problems and drawbacks are present.
First the organic absorbent, although it could be a very selective solvent for maleic anhydride and rather hydrophobic (the mutual solubilities DBP-water are very low), all the same a moderate absorption of water occurs inside the absorber. The absorption of water in dibutylphthalate occurs in presence of maleic anhydride in liquid phase.
Under the above mentioned circumstances the absorption of water in DBP is not only controlled by the simple liquid-vapor equilibrium rules. It is know in the art that aqueous contact cause the conversion of part or most of the maleic anhydride to maleic acid. In this mechanism, an undesirable side reaction can also take place, consisting in the concurrent isomerization of part of the maleic acid to fumaric acid. In conclusion the absorption of water in DBP and in presence of maleic anhydride/acid does not depend only from the temperature and the pressure; a likewise important parameter is the time in which the two components are in contact.
It is important to note that the absorbed water not only causes that part of maleic anhydride is converted to fumaric acid, i.e. a loss of product, but also it promotes the deposits of solids fumaric acid, characterized by a very limited solubility in water or in organic solvents, inside the internals of the columns, vessels, heat exchangers, pumps and pipes of the plant. Of course such solubility depends from the temperature, with lower values as lower is the temperature.
A second aspect of the art of the maleic anhydride absorption in an organic solvent, concerns the heat removal system.
The effluent from the oxidation reaction is well above 400° C. This reaction gas typically contains only a small amount of maleic anhydride, i.e. from 0.6 to 1.2 percent by volume, together with other components, mainly nitrogen and residual oxygen, a certain amount of not converted n-butane and with the oxidation by-products, including carbon monoxide and dioxide, water, acetic acid, acrylic acid and other minor impurities. Before to enter in the absorption column to recover the maleic anhydride, the reaction product gas is sent to one or more heat exchangers, with production of medium pressure steam and pre-heating of the relevant boiler feed water, preferably below 200° C., anyway above the dew point of the water produced in the oxidation reaction and contained in the fresh air used for the oxidation. Generally such temperature is within the range of 130 to 170° C.
The reaction gas in conclusion is characterized by a very high flowrate (from 20,000 to 160,000 Kg/h for a single commercial maleic anhydride reactor) and by a rather high temperature. It means the amount of the heat to be removed in the absorber to produce an efficient recovery of the product is very high. The heat capacity of the fresh solvent, unless to use a very huge and uneconomical amount, is not sufficient to absorb of the necessary heat load.
A third aspect not comprised in the prior art is related to the hydraulic of the internals of the absorption column.
It represents actually a typical gas/liquid contact equipment, where the volumetric ratio gas to liquid is extremely high. Who is skilled in the art of designing this type of column is familiar with the difficulties of reaching high levels of absorption efficiency.
A fourth and last aspect refers to the combination absorber/vacuum stripper. As a matter of fact the absorption solvent sent to the top of the maleic anhydride recovery column is used in a closed loop, where the maleic anhydride is removed by the enriched solvent in a reboiled column operated at very low pressure. In spite of the low pressure of this stripper and in consideration of the tendency the decomposition of the DBP at temperature higher than 200° C., it is basically impossible to completely remove the maleic anhydride from the stripped solvent such poor solvent, after adequate cooling, is recycled back to the top of the absorption tower. It means in the overhead tray of the column the gas leaving from the tray below, at this point practically without or with very low maleic content, contacts again a stream of solvent containing still maleic anhydride. It is a logical result that such maleic anhydride is partially stripped out from the solvent and it is lost in the overhead gas, typically sent to a thermal oxidizer.
As final result in the state of art, even if the removal efficiency of the absorber could be high, f.e. by using a huge number of trays of by using a huge amount of solvent, the overall recovery efficiency is always limited by the above described strip-out of product in the top tray of the absorber.
Accordingly to the above described four points, there exists a need for an improved process and apparatus for the absorption column of maleic anhydride in an organic solvent, in which the overall yield of recovery is increased and the formation of fumaric acid deposits is decreased.